So even if Luffy was tought in haki usage, I don't think he would have an easy time against neither Magma, Ice nor Light nor Sengoku's Buddha fist that punched him out of Gear 3 Fuusen. Not even experienced fighters like Marco, Jozu and Vista were able to cause more than some nuisance to logia users.
I actually believe we've seen Luffy using haki way before even hearing its name. Remember when Blueno said something about that Luffy's Gomu Gomu no Rifle in Enies Lobby was much stronger than his kick back in the Galley-La headquarters since it broke through his Tekkai? There's some panel right before Luffy's attack connected (
Ch.388 p.12 - upper left panel) that made me curious. There is a similar one when he was about to hit Blackbeard (
Ch. 544 p.04 - mid-right panel) - some moments before Blackbeard mentions that
Luffy's haki has improved.
How could haki usage be more or less boring than
insert-random-df-power? It's like anything else in One Piece a tool to make the story progress how Oda intends it to go on.